The present invention relates a group modulator, which is used in a digital radio communication system, for providing a plurality of modulated signals based upon a plurality of input data.
In a mobile communication system, a cellular communication system, and/or a personal communication system, a base station must transmit a plurality of radio signals simultaneously. The number of radio signals in each base station is lately increasing because of increase of traffic.
FIG. 1 shows a spectrum of modulated radio signals which are transmitted simultaneously, where f1 through f5 are center frequency of each radio signal. The solid lines f1, f3 and f4 show radio signal which is actually modulated, and the dotted lines f2 and f5 show the status that the modulated signals are not output.
A digitally modulated signal which has a carrier signal modulated with base band data signal in digital form is produced by using a quadrature modurator which receives in-phase component (I-channel signal) and quadrature component (Q-channel signal).
FIG. 2 shows a block diagram of a prior modulator which provides a plurality of radio signals. In the figure, the numerals 100, 101 and 102 show a modulator which modulates a single input data, and 103 is an adder for adding the outputs of all the modulators. The output of the adder 103 is high frequency signal. As the structure of the modulators 100 through 102 is the same as one another, the modulator 100 is described in detail. The modulator 100 has a mapping circuit 112 which receives an input data #1, and provides I-channel signal and Q-channel signal for quadrature modulation according to instantaneous pattern of input data, and modulation system. Those I-channel signal and Q-channel signal are applied to the pulse shaping circuit 113 which restricts bandwidth of those signal. The I-channel signal and the Q-channel signal of the output of the pulse shaping circuit 113 are then applied to the digital-to-analog converters 131 and 132, respectively for the conversion of the signal from digital form to analog form. The outputs of the converters are applied to the mixers 151 and 152 which are a part of the quadrature modulator, through the low pass filters 141 and 142, respectively. The oscillator 155 provides the carrier frequency f1 which is designated by an external carrier designation signal #1. The carrier frequency f1 of the oscillator 155 is applied to the mixer or the multiplier 151 and the other mixer or the multiplier 152 through the /2 phase shifter 154. The outputs of the mixers 151 and 152 are added in the adder 153, which provides the quadrature modulated signal which has the carrier frequency f1 modulated with the input data #1. Similarly, the modulators 101 and 102 provide the quadrature modulated signals having the carrier frequencies f2 and f3, respectively, modulated with input data #2 and #3, respectively. The combination of the mixers 151 and 152, the adder 153, the oscillator 155 and the phase shifter 154 is a conventional quadrature modulator. Those modulated signals are added in the adder 103, to provide high frequency radio signal.
It should be noted in FIG. 2 that each input signal is modulated in each modulator, in other words, a plurality of modulators equal to the number of input data are essential.
Therefore, the prior art of FIG. 2 has the disadvantages that the structure of the apparatus is complicated as a plurality of modulators are essential, and that the amount of the traffic to be handled is limited by a number of modulators installed, in other words, it is impossible to follow adaptively the change of amount of traffic of input data.
In order to solve those disadvantages, we first considered the group modulator shown in FIG. 4, which was considered in our research laboratory, but is not commercially used. That is now explained.
The frequency f.sub.i is considered that the reference carrier frequency f.sub.c (=w.sub.c /2 ; w.sub.c is angular frequency, is pi=90.degree.) shifts by .DELTA.f.sub.i (=.DELTA.w.sub.i /2 ), as shown in FIG. 3
Digitally modulated signal S(t) is expressed as follows. EQU S(t)=A(t) cos(.THETA..sub.i (t)+(w.sub.c .DELTA.w.sub.i)t) (1)
where A(t) is instantaneous value of base band signal in signal space diagram, and .THETA..sub.i (t) is instantaneous phase angle.
The equation (1) is transformed as follows. EQU S(t)=A(t) cos (.THETA..sub.i (t)+.DELTA.w.sub.i t)cos w.sub.c t -A(t) sin (.THETA..sub.i (t)+.DELTA.w.sub.i t)sin w.sub.c t (2)
Therefore, it should be noted that a modulated signal having the carrier angular frequency (w.sub.c +.DELTA.w.sub.i) may be produced by (1) frequency-shifting a base band signal .THETA..sub.i (I-channel signal and Q-channel signal, respectively) by .DELTA.w.sub.i, and (2) effecting quadrature modulation for angular carrier frequency w.sub.c with the frequency shifted signals. The resulated modulated signal is the same as the quadrature modulated signal of angular carrier frequency (w.sub.c +.DELTA.w.sub.i) with the non-shifted base band signal .THETA..sub.i (t).
A plurality of modulated signals having a plurality of carriers are expressed as follows. EQU S(t)=.SIGMA.A(t) cos (.THETA..sub.i (t)+.DELTA.w.sub.i t)cos w.sub.c t -.SIGMA.A(t) sin (.THETA..sub.i (t)+.DELTA.w.sub.i t)sin w.sub.c t (3)
Therefore, it should be noted that a plurality of modulated signals are obtained by the steps of (1) frequency-shifting I-channel signal and Q-channel signal of each base band signal .THETA..sub.i by the frequency .DELTA.w.sub.i (=2 f.sub.i) which is defined by the externally supplied carrier control signal, (2) adding all the I-channel signals (and all the Q-channel signals), and (3) effecting quadrature modulation for angular frequency w.sub.c with the sum of the I-channel signals and the sum of the Q-channel signals.
FIG. 4 shows a block diagram of the prior group modulator implementing above consideration. In the figure, the symbols 61, 62, and 6n are a base band signal process circuit each of which receives an input data #1, #2, and #n, respectively. Each base band signal process circuit functions to serial-parallel conversion of an input data, mapping of signals allocating amplitude and phase of I-channel signal and Q-channel signal according to the pattern of the input data and the modulation system, to restrict bandwidth of the signals according to the transmission system, and to provide the frequency-shifted I-channel signal and Q-channel signal according to the externally supplied carrier control signal. The full adder 71 adds the I-channel signals of all the input data #1 through #n to provide the frequency-division multiplexed I-channel signal. Similarly, the full adder 72 adds the Q-channel signals of all the input data #1 through #n to provide the frequency-division multiplexed Q-channel signal.
The I-channel signal multiplexed by the full adder 71 is converted to analog form by the digital-to-analog (D/A) converter 31, the output of which is applied to the quadrature modulator 5 through the low pass filter 41, which restricts the bandwidth of the I-channel signal. Similarly, the Q-channel data multiplexed by the full adder 72 is converted to analog form by the D/A converter 32, the output of which is applied, through the low pass filter 42, to the quadrature modulator 5.
In the quadrature modulator 5, the multiplier 51 provides the product of the output of the low pass filter 41 and the carrier frequency f.sub.c which is supplied by the oscillator 55, and the multiplier 52 provides the product of the output of the low pass filter 42 and the carrier frequency f.sub.c with the phase shift by /2 supplied by the oscillator 55 through the phase shifter 54. The adder 53 adds the outputs of the multipliers 51 and 52 to provide radio frequency signal RF, or intermediate frequency signal IF.
However, the prior apparatus of FIG. 4 has the disadvantage that a plurality of base band signal process circuits 61 through 6n must be essential, since each input data is separately processed to provide I-channel signal and Q-channel signal for the shift frequency .DELTA.w.sub.i, so that a plurality of modulated signals each having specific carrier frequency are provided by adding all the I-channel signals and all the Q-channel signals.
Further, the number of input data is restricted to the number of the baseband signal process circuits.
The prior apparatus has further disadvantage that some of the baseband signal process circuits would not operate when the number of input data is less than the number of the baseband signal process circuits. It should be appreciated that the number of input data is adaptive based upon instantaneous amount of traffic. Thus, the prior apparatus has the disadvantage that the system does not follow the increase and/or the decrease of the traffic.